Motor overload tripping system and method with multi-function circuit interrupter

ABSTRACT

A technique is provided for controlling operation of motors of different sized or ratings. Components used to apply and interrupt current to the motors may be shared in control devices for the different motors. The components may include contactors or circuit interrupters, and instantaneous trip devices. The components may be sized for the higher rated motors, and be oversized for the lower rated motors. Control circuitry permits the devices to be controlled in accordance with the characteristics of the particular motor to which the devices are applied, providing accurate circuit interruption while reducing the number of different components and component packages in a product family for the various motors.

The present application is based on and claims the benefit of Chinesepatent application Ser. No. 2003101001836, filed Oct. 16, 2003, thecontents of which are hereby incorporated by reference in theirentirety.

BACKGROUND

The present invention relates generally to the field of electricprotective devices, such as for providing overload protection for motorsand other loads.

In the field of electrical protective devices, a range of devices areknown and are presently in use in various combinations. For example, ina typical arrangement for applying power to a load, such as an electricmotor, the circuitry may include fuses, circuit breakers, thermaloverload tripping devices, contactors, and so forth. In a typicalapplication, the components are selected and assembled based upon thecurrent ratings and other operating parameters of the load. For example,motor protective circuitry is typically selected based upon the motorfull load current rating. A thermal overload device may be selected toprovide tripping at a first level, such as 6 to 10 times the full loadof current rating, with faster, higher current tripping being providedby an electromagnetic device, such as a circuit breaker.

While such arrangements provide adequate protection for loads, they arenot without drawbacks. For example, many separate and differently ratedcomponents are generally provided and associated with one another invarious combinations, depending upon the nature and rating of the load.Little effort has been made in the field for reducing the number ofcomponents or the number of product offering by extending the ratings ofthe components and circuits. There is a need, however, in the field forproduct offerings that can service a range of loads, such as motors ofdifferent sizes, while providing both thermal and instantaneous tripperformance for larger loads, and smaller loads equally. Such productsare not currently available on the market.

BRIEF DESCRIPTION

The present invention provides a novel approach to the design andimplementation of power delivery and circuit protection designed torespond to such needs. The technique of the invention permits areduction in the number of different components needed to providecontrol of motors of different sizes and ratings. In particular, circuitinterrupting components, such as a contactor and an instantaneous tripdevice may be selected based upon a higher rated current (e.g., of alarger machine), and used for smaller machines as well. Controlcircuitry is then configured to cause tripping or interruption ofcurrent to either the smaller or the larger motor, depending upon whichmotor is coupled to the device. In other words, the components may besized for the rating of a higher rated machine, and overrated for thesmaller machines. However, algorithms implemented by the controlcircuitry permit interruption of current to the smaller machines via thesame components.

DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a diagrammatical view of an electrical device protectivecircuit coupled to line power and to a network for selectivelyinterrupting current to motors of different classes, ratings or size;

FIG. 2 is a diagrammatical representation of certain of the controlcircuitry associated with the arrangement of FIG. 1;

FIG. 3 is a diagrammatical representation, providing additional detail,of certain of the circuitry illustrated in FIG. 2;

FIG. 4 is a diagrammatical representation of electrical circuitry forconditioning signals in the arrangements of FIGS. 2 and 3;

FIG. 5 is a diagrammatical representation of wire and motor protectivecircuitry for use in the arrangements of FIGS. 2 and 3;

FIG. 6 is a graphical representation of motor trip curves illustrating amanner in which the circuitry of the preceding figures may operate toextend a range of tripping and to apply algorithms specifically designedfor extending the range; and

FIG. 7 is a flow chart illustrating exemplary logic in the design andimplementation of an extended range component protection device inaccordance with aspects of the present technique.

DETAILED DESCRIPTION

Turning now to the drawings, and referring first to FIG. 1, a protectivedevice or system 10 is illustrated diagrammatically for providing powerto and for protecting a loads, such as an electric motors of differentclass, rating, or size 12, 12′, or 12″. In the illustrated embodiment,the device 10 is a three-phase device configured to deliver athree-phase power to the motor from power conductors 14, 16 and 18 whichare typically coupled to the power grid. In a typical application, theprotective device 10 can provide power directly from the grid to themotor, or the device may be configured for providing controlled power tothe motor, such as for soft starting the motor or for driving the motorat variable frequencies (i.e. an inverter drive). Device 10 is alsocoupled to a network 20, such as an industrial control and monitoringnetwork. Such networks may operate in accordance with any suitablenetwork protocol such as well-known DeviceNet, ControlNet protocols orany other suitable protocol. As will be appreciated by those skilled inthe art, such networks typically permit for the exchange of data betweennetworked devices. In the present context, a remote control andmonitoring circuit 22, such as a programmable logic controller, remotecomputer, or any other suitable device, may be coupled to the networkfor monitoring operation of the protective device 10, resetting thedevice, where appropriate, and so forth. Interactions between the remotecontrol and monitoring circuit 22 and device 10 will be discussed ingreater detail below.

In the embodiment illustrated in FIG. 1, device 10 includes aninstantaneous trip device 24, such as an electromagnetic overloaddevice. As discussed in greater detail below, the device 10 isconfigured to service a wide range of loads, much wider thanconventional motor starters and protective circuitry. Accordingly, andalso as described in greater detail below, the instantaneous trip device24 is selected to open the circuits between the power conductors 14, 16and 18 and the load at a substantially high current, as compared to thenormal full load current of certain of the devices that can be coupledto the protective device as loads. Protective device 10 further includesa contactor 26 which can also serve to open the conductive paths betweenthe phase conductors and the load, under the control of controlcircuitry 28. As will be discussed in greater detail below, the controlcircuitry 28 may regulate opening and closing of the contactor 26 indifferent current ranges, depending upon the rating of the variousmotors of different class, rating, or size 12, 12′, or 12″ to which thedevice is coupled. That is, below certain current ratings, the controlcircuitry 28 may cause the contactor 26 to open, thereby tripping thedevice, based upon a first algorithm as determined by the rating of theload. In a higher current range, the control circuitry 28 causes thecontactor to open to protect the conductors used to link the phaseconductors to the load. In this range of operation, the controlcircuitry 28 and contactor 26 effectively implement and instantaneoustrip regime via software.

The control circuitry 28 is preferably linked to the network 20 by anetwork interface 30. The network interface 30 may provide for signalconditioning, power for certain of the circuitry of the controlcircuitry 28, and generally serves to interface the control circuitryvia the network protocol with other devices on the network 20. Inparticular, the network interface 30 may permit resetting of thecontactor 26 remotely, such as by control signals received from theremote control and monitoring 22. The protective device 10 furtherincludes sensors, as indicated at reference numeral 32. In a presentembodiment, sensors 32 are current sensors, such as currenttransformers. Other types of sensors, may, of course, be employed,particularly for sensing currents applied to the motor 12. Inappropriate situations, sensors 32 may also include voltage sensors. Thesensors may operate in accordance with any suitable physical phenomenonsuch as Hall-effect sensors.

As noted above, the protector device 10, and particularly the controlcircuitry 28, in conjunction with the contactor 26 and the instantaneoustrip 24, permit application of power to the load coupled to the device.In accordance with aspects of the present technique, two separate typesof algorithms or controlled methodologies are implemented. In a firstmethodology, a trip range is defined below a desired multiple of themotor full load current rating. Above this full load current ratingmultiple, a separate and parallel algorithm permits tripping thatimitates an instantaneous trip device. The instantaneous trip device 24may thus be selected for a highest full load current in a range ofdevices to which the protector device 10 is designed to operate.However, because this multiple may be much higher than desired forcertain of the devices to which the protective device 10 is coupled, thealgorithm causes trips at a lower current multiple within the extendedrange.

The particular operation of the control circuitry designed to permitsuch operation is described in greater detail below. However, it shouldbe noted here that the preferred algorithms for operation of the controlcircuitry in a present embodiment permit the use of smaller wire thanhas previously been employed for many applications for which theprotective device is designed. That is, modeling and algorithm designdescribed below is particularly adapted to permit the use of 16 AWG wirefor conductors coupling the protective device 10 to the grid conductors,and for conductors extending to the load. It has been found that the useof 16 AWG wire greatly facilitates installation and servicing of thedevice. Such standardization was heretofore impossible given the ratingsof devices used for larger loads.

FIG. 2 illustrates certain functional circuitry of the control circuitry28. As indicated above, the control circuitry implements algorithms forprotection of the load to which the device is coupled. In particular,the circuitry implements a wire protection path 34 and a load or motorprotection path 36. While the instantaneous trip device 24 (see FIG. 1)is provided for tripping at very high currents or full load currentmultiples (the particular multiple depending upon the rating of thedevice to which the circuitry is coupled), the instantaneous trip device24 is preferably selected based upon the highest current rating of thefamily of devices to which the protective device 10 is designed to becoupled. That is, for devices with a higher current rating, theinstantaneous trip device will provide a lower full load currentmultiple trip point. For lower-rated devices, however, the instantaneoustrip device will provide a much higher full load current rating multiplefor tripping. To accommodate this situation, the control circuitry 28illustrated in FIG. 2, permits protection of the motor and wiring in afirst range of operation, and particularly protects the wiring in anextended range over which an instantaneous trip device would operate inconventional arrangements.

In the diagrammatical view of FIG. 2, the wire protection path 34 andmotor protection path 36 are coupled at downstream of a rectifiercircuit 38 which receives input from the current sensors 32. To permitthe use of certain current sensors and the extended overload range, anuisance trip avoidance circuit 40 is provided downstream of the wireprotection path 34. That is, as will be appreciated by those skilled inthe art, signals from the current sensors may degrade at higher currentlevels. Thus, time constants used in the models implemented by thecircuitry (discussed in greater detail below) may provide for fastertripping than in conventional devices. Such faster tripping, then, willaffect the tripping at lower currents and could cause nuisance tripping.Such nuisance tripping can result from motor asymmetry transients,particularly upon startup, as discussed in greater detail below, thenuisance trip avoidance circuitry 40 permits the use of time constantsthat would otherwise result in faster tripping, while avoiding nuisancetripping due to such asymmetries.

In the diagrammatical representation of FIG. 2, the wire protection path34 and the motor protection path 36 appear to be generally similar. Thatis, the wire protection path 34 includes signal condition circuitry 42and wire thermal protection circuitry 44. Similarly, the motorprotection path 36 includes signal conditioning circuitry 46 and motorthermal protection circuitry 48. As discussed below, the circuitry, infact, act on different signal inputs from the rectifier circuitry 38,and model heating of the wiring and load in different manners, and basedupon different input parameters. Tripping of the contactor 26 (seeFIG. 1) may be based upon either the modeling provided by the wireprotection path 34 or the motor protection path 36. This permitsimplementation of algorithms for tripping in the two separate ranges asdiscussed in greater detail below. The arrangement also permits the useof smaller wiring (e.g. 16 AWG) and standardization upon the desiredwiring. In the embodiment illustrated in FIG. 2, output of the nuisancetrip avoidance circuitry 40 and of the motor thermal protectioncircuitry 48 are combined in an “OR” device 50 which produces a tripsignal for the contactor 26.

A present implementation of the circuitry illustrated generally in FIG.2 is shown in FIG. 3. As noted above, the wire protection path 34 andthe motor protection path 36 are coupled to the rectifier circuitry 38which receives signals from the current sensors. The wire protectionpath 34 then includes peak detection/buffer circuitry 52, scalingcircuitry 54, and further scaling circuitry 56. Implementations of theillustrated circuitry are more fully illustrated in FIG. 4 discussedbelow.

Based upon the peak detected current, which is scaled by circuitry 54and 56, the wire thermal protection circuitry 44 receives a scaledcurrent input and models wiring heating via wiring thermal modelingcircuitry 58. Circuitry 58 estimates heating of the wiring that suppliespower to the load based upon an assumed thermal constant or “τ” asindicated at reference numeral 60 in FIG. 3. As will appreciated bythose skilled in the art, the value of τ can be used as the basis for alogarithmic heating function that relates current to the temperature ofthe wiring. A heat value calculation circuit 62, then, estimates adesired or rated temperature or heat value for the wiring. The modeledwiring temperature and the heat value calculation circuit temperatureare then compared at a comparator 64. When the estimated or modeledwiring temperature approaches or exceeds the desired wiring temperature,a trip signal is generated by the comparator 64. This trip signal is,however, fed to the nuisance trip avoidance circuitry 40 prior to beingapplied to the “OR” device 50. Thus, nuisance trips based upon higherpeak occurrence do, for example, to motor asymmetries, are avoided.

The motor protection path 36 includes signal conditioning circuitry 66that receives input from the rectifier circuitry 38. The signalconditioning circuitry 66 is also described in greater detail below withreference to FIG. 4 in a present implementation. Based upon conditioningprovided by the circuitry 66, output signals are compared at acomparator 68, to produce a single output signal which is scaled byscaling circuitry 70. The scale signal is then applied to a motorthermal modeling circuitry 72 of the motor thermal protection circuitry48. In a manner similar to that of the wiring thermal modeling circuitry58, the motor thermal modeling circuitry 72 estimates heating of theload based upon and assumed thermal time constant τ. The thermal timeconstant is input as indicated at reference numeral 74. As will beappreciated by those skilled in the art, different thermal timeconstants may be provided for different loads, i.e., different motorshaving different ratings. The resulting modeled heating is then comparedto anticipated or desired heating computed by a heat (trip) valuecalculation circuit 76. The computed trip value is itself computed basedupon the rated full load current for the motor coupled to the circuitry.Based upon this comparison, performed by a comparator 78, an output ortrip signal is generated that is applied to the “OR” device 50. As alsoillustrated in FIG. 3, a reporting or indicator signal is generated by acomparator 80 that is a ratio of the modeled temperature to the desiredtemperature. This signal may be applied to downstream circuitry, such asto remote control or monitoring circuitry 22 (see FIG. 1). The outputsignal may provide an indication of motor heating as a function of thedesired or rated heating.

FIG. 4 is diagrammatical illustration of certain of the upstreamcircuitry for signal conditioning illustrated in FIG. 3. In particular,as illustrated in FIG. 4, inputs from the current sensors may be appliedto inductors 82 (if appropriate) and then to the peak detection/buffercircuitry 52 and to the signal condition circuitry 66. The peakdetection/buffer circuitry 52 includes a series of resistors, operationamplifiers, and diodes that serve to buffer the input signals andprovide a single peak output applied to the scaling circuitry 54.Scaling circuitry 54 affectively scales the output of the peakdetection/buffer circuitry 52, which may be on the order of 24 VDC,producing a scaled output of 0 to 5 VDC. The circuitry 52 and 54affectively account for signal degradation that may occur at highercurrents. That is, in certain cases, and depending upon the types ofcurrent sensors used, signal degradation may occur due to saturation ofthe sensor components. The circuitry permits the use of such sensors,however, despite the considerably extended current range of theprotection device as described herein. As will be appreciated by thoseskilled in the art, for example, each current transformer will producewaveforms similar to those illustrated by the graphical representation84 in FIG. 4. Output of the peak detection/buffer circuitry 52 andscaling circuitry 54 will, however, provide a slightly rippled outputwaveform as indicated by the graphical representation of 86 in FIG. 4.

The signal conditioning circuitry 66 illustrated in FIG. 4 comprises aseries of resistors and capacitors. The circuitry permits for reductionof noise due to electromagnetic interference, as well as protection fromoverdriving analog 2-digitial converters of the motor thermal protectioncircuitry 48. In a present embodiment, the signal conditioning circuitry66 produces output signals ranging from 0 to 5 VDC.

In a present embodiment, the circuitry illustrated in FIG. 4 is providedon a first printed circuit board, while a second print circuit boardsupports the scaling circuitry 56, comparator circuitry 68, scalingcircuitry 70, and the wiring and motor thermal modeling circuitry (see,e.g., FIG. 3). Any other suitable construction or topography may, ofcourse, be employed. Indeed, the present device is particularlywell-suited for application with and mounting in close proximity to aload to which power is applied and which is protected by the circuitry.Such configurations, which may be referred to as “on-machine”configurations, provide for application of power adjacent to the load,while providing the highly-adaptive control and protection functions ofnetworked control systems. The present techniques are not, however,limited to on-machine implementations.

The foregoing circuitry is illustrated in somewhat greater detail inFIG. 5. As noted above, the circuitry of FIG. 5 is, in a presentembodiment, populated on a single control circuit board. The wirethermal protection circuitry 44 receives input from signal conditioningcircuitry that, in turn, receives input from the scaling circuitry 54discussed above. In a present implementation, scaling is performed foran i²t inverse time modeling algorithm as indicated at reference numeral88. Scaling is performed by a scaling divisor 90 which generates a ratioof the signal received from scaling circuitry 54 and the input from thei²t scaling module 88. This scaled input is then applied to the wiringthermal modeling circuitry 58. As noted above, based upon the modeledheating of the wiring (e.g., standardized 16 AWG), as dictated by thetime constant τ inputted reference numeral 60, and the comparisonperformed by comparator 64, a trip signal may be generated based uponwire heating.

Similarly, output from the signal conditioning circuitry 66 discussedabove is applied to comparator 68 of the scaling circuitry 70. Againbased upon an i²t scaling modules 92, a scaling signal is applied to ascaling divisor 94 which generates a scale signal which is a ratio ofthe inputs. This scale signal is then applied to the motor thermalmodeling circuitry 72. Based upon the time constant τ input as indicatedat reference numeral 74, and the comparison made by comparator 78, then,a trip signal may be similarly generated based upon modeled motorheating.

As noted above, to permit the use of certain types of current sensors,and to account for asymmetric transients in the load (e.g., uponstarting) nuisance trip avoidance circuitry 40 is provided. In theimplementation illustrated in FIG. 5, a circuitry includes a tripthreshold input which is a number of counts, such as 20 counts. A tripcount input 98 is also consecutive trips provided that is a runningcount of the number of consecutive trip signals generated by thecomparator 64. In the present implimentation, then, samples of the tripsignals generated by comparator 64 are accumulated based upon one msinterrupts. If the value measured is above a predetermined constantvalue, such as 9 times the permitted wire current, the trip count 98 isincremented. This incremented value is then compared to the tripthreshold, such as the constant of 20 counts by a comparator 100. Thus,if 20 counts above the desired threshold are accumulated, an enablesignal is output by the comparator 100 to and “AND” device 102. Thenuisance trip avoidance circuitry 40, thus, requires that a constantelevated current level is detected for the programmed time (e.g., 20 ms)and that the trip signal from comparator 64 is present in order togenerate a trip output to be applied to the “OR” device 50. The nuisancetrip avoidance circuitry 40 permits the use of a τ value input (seereference numeral 60) that provides for fast tripping and the use ofcertain types of current sensors. The circuitry also facilitates use ofa threshold enabling an extended range of (instantaneous) overloadtripping with the use of the same contactor for both large and smallloads.

In particular, the present arrangement facilitates the modeling ofheating for both overload protection and instantaneous tripping. Suchtripping is provided by the algorithms employed and implemented by theforegoing circuitry, which may be graphically illustrated as shown inFIG. 6. FIG. 6 illustrates two separate ranges of operation, including afirst range 106 designed to protect wiring and the load based uponconventional i²t inverse time algorithms as generally known in the art.However, the algorithms here are implemented by the same devices for anumber of different types and sizes of loads. A second, extended range108 is provided for higher current level tripping. The single or unifiedcurve of the extended range 108 is typically different than the curvesthat would be extended from the curves of range 106.

The graphical illustration of the ranges 106 and 108 of FIG. 6 areillustrated graphically along a horizontal axis 110 that representscurrent, and a vertical axis 112 that represents time. A boundarybetween ranges 106 and 108, as indicated generally at reference numeral114 may be defined by applicable electrical codes, such as at a level of6-10 times the full load current (FLC) for a particular class of motor.The multiple of the FLC defining the boundary 114 may be based upon thelocked rotor current, for example. An upper limit 116 for the extendedrange 108 is defined by the rating of the instantaneous trip device 24illustrated in FIG. 1 above. While the instantaneous trip device is, inconventional systems, typically selected as a circuit breaker designedto operate at a somewhat higher multiple of the FLC than the overloadtrip threshold, typically on the order of 13 times FLC, it will be notedthat in the present arrangement, because the instantaneous trip devicesselected based upon the highest FLC of devices to be serviced by theprotection circuitry, the multiple of the FLC for smaller loads will beconsiderably higher than in conventional devices. The foregoingcircuitry, thus, provides instantaneous tripping to accommodate for thisconsiderably extended range of operation. The particular size and ratingfor the instantaneous trip device (i.e. circuit breaker) may be selectedbased upon additional factors, such as minimization of the number ofdifferent components in the systems, and so forth, and may provide amultiple of the highest FLC greater or less than 13× (e.g. 20×).

In a typical implementation, standard curves defining the relationshipsof range 106 will be provided in a conventional manner. Such curves,which are typically defined by a class (e.g., class 10) provide formotor thermal protection up to the desired multiple of a full load ofFLC. An extended operation curve, indicated at reference numeral 120 inFIG. 6, then, defines operation of the device above the threshold 114,and provides for protection of the wiring in the event of a rapid buthigh current trip event. As will be appreciated by those skilled in theart, the algorithm resulting in the curve 120 will typically not modelthe trip current performance in a manner similar to the extension of thecurves in range 106. For example, in a case of a class 10 device, theextension of the corresponding curve from range 106 may be indicated bythe curve of reference numeral 122. However, the actual curve 120 may beshallower than the ideal i²t curve extended as indicated at referencenumeral 122. However, tripping is provided in the extended range by thecircuitry described above to protect wiring above the threshold 114.

The nature of the operation of the foregoing circuitry, as graphicallyillustrated in FIG. 6 may be summed up through series of considerations.First, the operating range 106 is defined by multiple curves based uponthe motor FLC. Such curves are typically dictated by class standards.The protection within this region or range is afforded for both themotor and the wiring. The circuitry thus recognizes the occurrence ofoverload conditions and opens the contactor 26 under the control of thecontrol circuitry 28 (see, e.g., FIG. 1).

Within the operating range 108, on the other hand, a single curve orrelationship is provided for tripping. Protection is thus afforded forthe smallest wire in the motor branch circuitry, which in a presentembodiment is selected as 16 AWG. The extended range similarlyrecognizes overload conditions which may be adversely affect the wire bysuch heating and causes opening of the contactor.

The following is an example the extended range operation of the presenttechnique. A single device, power and protection may be provided for arange of motors of a frame size C. The present technique provides foraccommodating motors from approximately 2 Hp to approximately 10 Hp,having minimum FLC ratings of 3.2 and maximum FLC ratings of 16Arespectively. Current sensing hardware, including current sensors,amplifiers and analog-to-digital converters, are provided for a range ofoperation to approximately 8 times the maximum of FLC (8×16A=128A). Thatis, the unit is designed to operate for overload conditions of up to 16A of the rated device, or an RMS current of 128A, with a peak ofapproximately 180A (128×√{square root over (2)}). Continuing with thisexample, the rating of approximately 180A will correspond to thethreshold 114 of FIG. 6. Tripping within the first range 106, however,will be determined by the circuitry described above and upon the timeconstants and curves implemented by the circuitry.

To provide for the extended range, an instantaneous trip device isselected based upon the highest FLC of the loads that can beaccommodated by the circuitry, in this example 16A. That is, the limit116 illustrated in FIG. 6 is effectively a fixed instantaneous triplevel as defined by the instantaneous trip device. In the currentexample the device may be selected for instantaneous tripping at amultiple of the highest FLC of the serviced devices, such as at 325ARMS. The peak operating condition, then, for this device would beapproximately 460A (325×√{square root over (2)}). It will be noted thatin the foregoing example, while a relatively standard multiple of theFLC of the larger serviced device is provided by the instantaneous tripdevice (approximately 20× the FLC of the largest frames C device, e.g.,a 10 Hp motor), a much higher multiple results for the smaller devices,on the order of 100× the FLC of the smaller device rated at 3.2A(325A/3.2A). However, due to the modeling and tripping provided by thecontrol circuitry and contactor described above, such extended rangesmay be afforded and multiple devices of substantially different currentratings may be serviced by the same hardware and software.

In a current implementation, for example, two different frames of motors(actually provided in the same physical frame) denoted frame A and frameB are serviced by a single device with FLC ranges of 0.5A to 5.5A. Asecond protective device is offered for a range of loads in a frame C,ranging from 3.2A to 16A as in the example discussed above. T (τ) values(provided in terms of τ times the sample period of 1 ms are set atvalues of 33, 78 and 262 for the three frames A, B and C, although suchvalues are highly dependent upon the time constant, sample rate scaling,trip levels, and other system and component design factors.

The methodology for design of the present protective devices, andfurther implementation is set forth generally in FIG. 7. Theimplementation logic, indicated generally by reference numeral 124, maybegin with selection of a contactor, such as contactor 26 illustrated inFIG. 1, based upon the highest instantaneous trip and full load currentrating for devices to be serviced by the protective circuitry. Assummarized above, although a range of devices will be serviced by theprotective circuitry, the use of the highest instantaneous trip and fullload current rating for the family of devices permits implementation ofan extended range algorithm that effectively protects wiring used forbranch circuitry coupled to the load.

As indicated at step 128, then, a program or model is implemented byclass for overload for a thermal overload tripping. Such programming isprovided in the circuitry described above, including the motor thermalprotection circuitry. The modeling provides for tripping below athreshold typically set by reference to an i²t inverse time algorithmfor a class of loads based upon a desired multiple, such as from 6 to 10times the FLC for the load. As indicated at step 130, the program ormodel is based upon the algorithm for wire and motor protection,typically the class standard algorithm. At the same time, nuisance tripavoidance is provided as indicated at reference numeral 132, toaccommodate for asymmetries in the load performance, typicallypermitting higher currents upon start up of a motor.

As indicated at step 134 in FIG. 7, instantaneous trip is then providedwithin the extended range discussed above. As also discussed above, theinstantaneous trip is typically provided based upon protection ofwiring, with standard wiring parameters being employed in a presentembodiment, such as to protect 16AWG wire. Reference numeral 136indicates the algorithm 2 that is employed in this trip regime for wireprotection as discussed above.

As noted above, the protective circuitry may be employed in a networksetting in conjunction with remote control and monitoring circuitry,such as circuitry 22 illustrated in FIG. 1. Where such circuitry isprovided, and due to the interfacing of the control circuitry 28 withthe network, remote reset capabilities are afforded, as indicated atreference numeral 138 in FIG. 7. Such remote resetting facilitatesmonitoring and continued operation of the devices, as opposed totraditional resetting which typically requires physical and manualpresence at the point of resetting. As noted above, because theforegoing circuitry is very well suited to mounting at a load location,or immediately adjacent to the locating of a load, such remote resettingis particularly advantageous as the loads may be widely displaced from acentral monitoring location.

While only certain features of the invention have been illustrated anddescribed herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

1. A system for controlling operation of an electric motor comprising: acontactor for selectively applying and interrupting electrical currentto a first electric motor and a second electric motor different from thefirst motor, having lower and higher ratings, respectively, thecontractor being rated for an electric motor with the higher currentrating and being generally overrated for an electric motor with a lowercurrent rating; and a control circuit coupled to the contactor, thecontrol circuit being configured to cause the contactor to interruptcurrent to the either the first or the second electric motor based uponthe current rating of the respective motor.
 2. The system of claim 1,wherein the control circuit is configured to emulate a thermal overloadcircuit rated for the first or the second electric motor.
 3. The systemof claim 1, wherein the control circuit is configured to implement ani²t inverse time algorithm adapted for either the first or the secondelectric motor.
 4. The system of claim 1, further comprising aninstantaneous trip device configured to interrupt current to the firstor the second electric motor.
 5. The system of claim 4, wherein theinstantaneous trip device is rated based upon the higher current rating.6. The system of claim 5, wherein the instantaneous trip device is ratedto interrupt current at approximately 20 times the full load currentrating of the electric motor with the higher current rating.
 7. Thesystem of claim 6, wherein the instantaneous trip device is rated tointerrupt current at approximately 100 times the full load currentrating of the electric motor with the lower current rating.
 8. Thesystem of claim 5, wherein the control circuit is configured to causeinterruption of current to the first or second electric motor via thecontactor up to a trip current rating of the instantaneous trip device.9. A system for controlling operation of an electric motor comprising: acontactor for selectively applying and interrupting electrical currentto a first electric motor and a second electric motor different from thefirst motor having lower and higher current ratings, respectively, thecontractor being rated for an electric motor with the higher currentrating and being generally overrated for an electric motor with a lowercurrent rating; a control circuit coupled to the contactor, the controlcircuit being configured to cause the contactor to interrupt current tothe either the first or the second electric motor based upon the ratedcurrent of the respective motor; and an instantaneous trip deviceconfigured to interrupt current to the first or the second electricmotor at a current level based upon the higher current rating.
 10. Thesystem of claim 9, wherein the control circuit is configured to emulatea thermal overload circuit rated for the first or the second electricmotor.
 11. The system of claim 9, wherein the control circuit isconfigured to implement an i²t inverse time algorithm adapted for eitherthe first or the second electric motor.
 12. The system of claim 9,wherein the instantaneous trip device is rated to interrupt current atapproximately 20 times the full load current rating of the electricmotor with the higher current rating.
 13. The system of claim 12,wherein the instantaneous trip device is rated to interrupt current atapproximately 100 times the full load current rating of the electricmotor with the lower current rating.
 14. The system of claim 9, whereinthe control circuit is configured to cause interruption of current tothe first or second electric motor via the contactor up to a tripcurrent rating of the instantaneous trip device.